OCT is a technique wherein imaging information can be obtained in the depth or z-direction of a sample, typically the retina of the eye. In conventional time domain OCT, the retina is scanned with a beam from an interferometer having a light source having a short coherence length, typically in the order of a few microns. A signal is obtained from the returned beam at depth positions wherein the optical path difference is less than the coherence length.
Different scanning techniques may be employed as described, for example, in U.S. Pat. No. 5,975,697, the contents of which are herein incorporated by reference. In the so-called A scan, the sample is scanned along a single axis in the depth direction to generate a reflectivity profile along the z axis at a particular point in the x-y plane. In a B scan, the sample is also scanned in either the x or y direction so as to generate a horizontal or vertical slice extending into the sample. The B-scan results from a succession of A scans. In en-face scanning, image slices in the x-y plane are taken at different depths.
In spectral OCT, described in U.S. Pat. No. 6,377,349 to Ferscher, the contents of which are herein incorporated by reference, and L. M. Smith and C. C. Dobson, Applied Optics, 1989, vol. 28, no. 15, pages 3339-3342, the spectrum of the light scattered by the object is obtained by a diode array in the object plane. In this case the optical A scan is obtained from a Fourier transform of the spectral intensity distribution of the light reflected by the object. Fourier transformation of the complex spectral amplitude gives information about the reflectivity of the sample along the z axis within the sample.
Spectral OCT is however subject to spectrometer losses and polarization effects, which can decrease the resolution obtainable. Instead of using a broadband source, it has been proposed to use a narrowband source, such as a laser, for the purpose of obtaining A scans. In this proposal, the frequency of the laser is modulated within a defined spectral band, and the response at each frequency within the spectral band recorded. Swept source scanning eliminates the need for a spectrometer since the different frequencies can be detected with a simple photodetector. Swept source scanning, however, has been limited to A scans, i.e. single axis scans extending in the depth direction, because it is difficult to obtain a stable frequency modulated source with a high sweep rate that would be required to perform a B scan (a sectional image extending in the depth direction). For example, assuming an image 1000 pixels wide and a frame rate of 1 frame/sec, the laser would need to sweep 1000 times/sec., which is very difficult to achieve in practice.